The present invention relates to a method and apparatus for photothermal destruction of toxic organic compounds, particularly those generated during combustion of waste. More particularly, it relates to a process which utilizes high temperatures in combination with radiation exposure to induce a photochemical reaction which completely detoxifies a wide variety of toxic organic compounds contained in a gas stream.
Throughout the industrialized world, there is a growing concern over the impact of industry on the environment, specifically with regard to the increasing emission of contaminants into the atmosphere. According to recent estimates, between 330 and 570 million tons of hazardous waste were generated between 1900 and 1980, and 290 million tons are now produced annually in the United States. Even larger quantities of municipal waste are produced and essentially every U.S. community has some sort of disposal facility or procedure. The level of concentration of such wastes pose health hazards to the general population.
One method of hazardous and municipal waste disposal has been the use of high temperature incineration. Incineration is capable of breaking down a wide variety of toxic materials into carbon dioxide, water, and simple mineral acids. However, this process is very expensive.
In addition, public concern over emission of undestroyed residual components, organic products of incomplete combustion, and off-gas streams laden with organic vapors such as dioxins (or more properly, polychlorinated dibenzo-p-dioxins, PCDDs), have led to regulations by the Environmental Protection Agency, which now requires that a hazardous waste incinerator owner/operator demonstrate a destruction and removal efficiency of 99.99% for the principle organic hazardous components in a waste stream. Polychlorinated biphenyls and polychlorinated dibenzo-p-dioxins are considered so toxic that an efficiency of 99.9999% is required for this class of compounds.
Measurements of emissions from municipal waste incinerators have shown that PCDD and related compounds such as polychlorinated dibenzofurans (PCDFs) are produced during the incineration process and are emitted in environmentally significant quantities. PCDDs are highly toxic chlorinated aromatic hydrocarbons and can include the highly toxic isomer 2,3,7,8 tetrachloro dibenzo-p-dioxin (2,3,7,8 TCDD). Continued concern over PCDD emissions from both hazardous and municipal waste incinerators has prompted the EPA to propose a 30 nanogram/distandard cubic meter emissions limit on all combustion sources as part of its new combustion strategy. Recent research suggests that dioxins are not formed in the combustion zones of the incinerator but are instead formed downstream in the air pollution control devices designed to control gases and particulate matter. Thus, control of dioxin emissions has been extremely difficult.
As a result, there has been extreme interest in developing non-incineration methods for waste treatment. One such method involves treating large volumes of toxic organic wastes with concentrated solar radiation. Sunlight contains both infrared radiation, which can be used to heat the waste, and ultraviolet radiation, which can promote photochemical reactions. However, the unreliability of solar energy limits its use in detoxification processes.
As a result, other more reliable sources of radiation such as high intensity ultraviolet lamps have been employed. For example, U.S. Pat. No. 4,981,650 to Brown et al, discloses the treatment of dioxin contaminated soil or liquid with an activating agent such as 1-hexadecylpyridinium chloride and an alcohol, fatty acid or fat, and then, irradiating with ultraviolet radiation. However, ambient temperatures are maintained during the process, which limits treatment capacity due to the prolonged radiation exposure times required to ensure thorough destruction of the toxic compounds.
Thus, while prior art processes have utilized either heat or radiation to destroy hazardous wastes, none of the processes have used a combination of intense heat and radiation. Although some processes disclose the use of slightly elevated temperatures as well as radiation, such slightly elevated temperatures are a physical requirement, e.g., for the purpose of preventing moisture condensation, and do not effectively aid in the detoxification process itself.
Accordingly, there is still a need in the art for a process which effectively destroys dioxins and other toxic compounds produced during a combustion process.